docs: def readInstance(self, key, makeGlyphs=True, makeKerning=True, makeInfo=True):
          """ Read a single instance element.
  
              key: an (attribute, value) tuple used to find the requested instance.
  
          ::
  
              <instance familyname="SuperFamily" filename="OutputNameInstance1.ufo" location="location-token-aaa" stylename="Regular">
  
          """
          attrib, value = key
          for instanceElement in self.root.findall('.instances/instance'):
              if instanceElement.attrib.get(attrib) == value:
                  self._readSingleInstanceElement(instanceElement, makeGlyphs=makeGlyphs, makeKerning=makeKerning, makeInfo=makeInfo)
                  return
          raise MutatorError("No instance found with key: (%s, %s)." % key)